Mobile communication systems allow users access to a communications network through respective user terminals as the user moves within a geographic region. Some mobile communications systems partition the geographic region into areas wherein each area may be covered by a portion of the mobile communications system. Consequently, if a user moves from a first area to a second area within the serviced geographic region, the mobile communications system may stop providing service in the first area and start providing the service in the second area to maintain the user's access. The users may access the mobile communications system using a user terminal.
FIG. 1A illustrates a satellite system which provides communication service to user terminals 200 located within areas 120 and 125 via respective spot beams 101 and 102. The satellite system provides communication service to the user terminal 200 by tracking the location of the user terminal 200 within areas 120 and 125 and registering the user terminal 200 within that area. For example, if the user terminal 200 were located in area 125, the satellite system would register the user terminal 200 as located in area 125 and communicate with the user terminal 200 via spot beam 102. If, however, the user terminal 200 were to relocate to area 120, the satellite system may re-register the user terminal 200 as located in area 120 and subsequently communicate with the user terminal 200 via spot beam 101.
Spot beams 101 and 102 may provide respective communications channels for communications between the satellite 110 and the user terminal 200. A spot beam may also provide respective control channels to user terminals within the area covered by the spot beam. A control channel may carry information concerning the service provided with the area covered by the respective spot beam. For example, spot beam 102 may broadcast information over a control channel which identifies spot beam 102 to the user terminals located within area 125, and may also identify communications channels assigned to the spot beam.
The location of the user terminal 200 may be registered via a location update procedure. The location update procedure is a communications protocol whereby the user terminal 200 may notify the satellite system of the user terminal's location within the geographic region. A location update procedure may be performed when the user terminal 200 moves to a new area and requires re-registration in the new area. The user terminal 200 may initiate the location update procedure when the user terminal 200 detects that the signal strength of its present spot beam is less powerful than that of another spot beam. For example, if the user terminal 200 were located within area 125 serviced by spot beam 102 and subsequently moved to area 120, the user terminal 200 may detect the greater signal strength of spot beam 101. Upon detecting the greater signal strength of the spot beam 101, the user terminal 200 switches from the control channel associated with spot beam 102 to the control channel associated with spot beam 101 and notifies the satellite system of its new location corresponding to area 120 using the information broadcast on the control channel associated with spot beam 101. Thus the user terminal 200 re-registers with the satellite system in area 120. Subsequently, the satellite system communicates with the user terminal 200 using spot beam 101. The user terminal 200 may also perform periodic location update procedures if the user terminal 200 operates in a particular location for a long time.
Some satellites may exhibit a behavior known as an inclined orbit wherein the spot beams projected onto the geographic region periodically shift or oscillate. Such an oscillation is illustrated in FIG. 1B. At a time t.sub.1, the satellite 110 services the area 120 via spot beam 101 and the area 125 via spot beam 102. The user terminal 200 is located within the area 125 serviced by spot beam 102. At a time t.sub.2, the inclined orbit causes an oscillation in the spot beams covering areas 120 and 125. Specifically, spot beam 101, formerly covering area 120, now covers area 120'. Similarly, the spot beam 102, formerly covering area 125, now covers area 125'. Moreover, user terminal 200, formerly located in area 125, is now located within the area 120' serviced by spot beam 101. Thus, the user terminal 200 has experienced a shift in spot beam service without moving. At a time t.sub.3, the coverage shown at time t.sub.1 is restored, causing yet another shift in the coverage of the spot beams and the service to the user terminal 200. The oscillation associated with an inclined orbit may be such that the shift in spot beam coverage described in FIG. 1B happens periodically.
As described above, the user terminal 200 may initiate location update procedures upon the detection of changes in the spot beam service. At time t.sub.1, for example, the user terminal 200 detects service via spot beam 102. At time t.sub.2, however, user terminal 200 detects a change such that its service is provided by spot beam 101. Consequently, the user terminal 200 may initiate a location update procedure. Subsequently, the user terminal 200 may detect another shift in spot beam service when the inclined orbit causes an oscillation in the reverse direction. Consequently, the user terminal may perform yet another location update. Moreover, a location update procedure may be requested for each periodic shift in the spot beam coverage.
The user terminal 200 may be a communications device, such as a radio telephone, which is capable of communicating with satellite system. The user terminal 200 may detect the signal strength of spot beams and a beam pair location update timer within the user terminal 200 may measure time associated with a change in spot beam service. A periodic location update timer, within the user terminal 200, may measure the time elapsed since the present spot beam began servicing the user terminal 200.
In some systems, the number of user terminals located within an area affected by the periodic oscillation resulting from the inclined orbit may be significant. For example, in some systems 10% of the user terminals serviced by the satellite system may be located in areas subject to oscillations in spot beam service. Consequently, 10% of the user terminals 200 serviced by satellite communications system may initiate location update procedures upon detecting each oscillation in spot beam service. Such a significant number of location update procedures may cause a significant strain on the satellite system's resources.
Existing systems may use a registration process to reduce the number of location updates produced by oscillations in spotbeam coverage. The registration process may require each spot beam to broadcast a single location area code (LAC) which uniquely identifies the spot beam within the satellite system and a list of neighboring spot beams called "beam pairs" on the corresponding control channel. The beam pairs may also be identified by a unique beam pair LAC. For example, spot beam 102 and spot beam 101 form a beam pair 115. The control channel for spot beam 102 may therefore carry the single LAC corresponding to spot beam 102 and a list of LACs each of which correspond to a pair of spot beams. One of the LACs within the list of beam pair LACs would correspond to the beam 115 formed by spot beam 101 and spot beam 102. The user terminal 200 may receive the single LAC and beam pair LAC list broadcast on the control channel and use the LACs to register with the satellite system. Subsequently, the satellite system locates the user terminal using the LAC with which the user terminal 200 is registered. Furthermore, user terminal 200 may store the single LAC and the list of spot beam pair LACs broadcast by its servicing spot beam. When the user terminal 200 detects a shift in its spot beam service, the user terminal 200 may refer to the stored single LAC and the list of beam pair LACs to determine whether a location update procedure is necessary.
If the stored list of spot beam pair LACs indicates that the new servicing spot beam forms a beam pair with the registered spot beam, the user terminal 200 may register with the spot beam pair. For example, if the user terminal 200 were to move from area 125 to area 120, the user terminal 200 would detect the change in coverage from spot beam 102 to 101. Furthermore, the user terminal 200 would examine the single LAC broadcast by the spot beam 102 and the stored list of beam pair LACs to determine that spot beam 101 forms a pair with spot beam 102. Consequently, the user terminal 200 would register with the satellite system as being serviced by the spot beam pair formed by spot beam 101 and spot beam 102. Subsequently, any requests for communication (i.e., pages) directed to the user terminal 200 would be broadcast both on spot beam 101 and spot beam 102.
When the user terminal 200 registers with its servicing beam pair, the user terminal 200 starts a beam pair location update timer. The beam pair location update timer's duration exceeds the period of oscillation associated with the inclined orbit. For example, if the inclined orbit were such that the period of oscillation were six hours, the duration of the beam pair location update timer 220 would be greater than six hours. If the user terminal does not detect a restoration of its original spot beam coverage before the expiration of the beam pair location update timer, the user terminal 200 registers with the single spot beam currently providing service. If, however, the user terminal detects a restoration of its original spot beam service, the user terminal maintains its current registration with the beam pair and re-initiates the beam pair location update timer. By maintaining the user terminal's registration with the beam pair, the system has determined that the user terminal is located in an area subject to oscillation resulting from the inclined orbit. Furthermore, if the user terminal remains within this area, it will experience a continuous oscillation in its spot beam service.
Notwithstanding the communications systems and methods discussed above, there is a need to further reduce the number of location update procedures which occur within satellite communications systems.